Pitting resistant carbon coating

ABSTRACT

A hydrogenated diamond-like coating (“H-DLC”) for metallic substrates provides improved reliability. The H-DLC is relatively soft and elastic. Unlike hard and/or inelastic coatings in the prior art, the present coatings do not exhibit a loss of adhesion (delamination). A bonding layer may be used between the metallic substrate and the H-DLC. H-DLC coatings can, for example, be used in bearings and gears to reduce the occurrence of micropits and, ultimately, product failure.

STATEMENT OF GOVERNMENT INTEREST

The United States Government has rights in the invention describedherein pursuant to Contract No. DE-AC02-06CH11357 between the UnitedStates Department of Energy and UChicago Argonne, LLC, as operator ofArgonne National Laboratory.

FIELD OF THE INVENTION

The present invention generally relates to carbon coatings.

BACKGROUND OF THE INVENTION

Pitting is a major source of failure in gear and bearing devices. Therepeated cycling of loads and changes in slide to roll ratios leads tothe formation of cracks on stressed surfaces. These initial crackspropagate in micro-pitting. This micro-pitting then grows over time toform macro-pitting, which typically results in component failure. Suchfailure is addressed in the prior art by using precisely controlledsurface roughness, lubricant and additives in lubricant, and use ofhigh-purity metallic components, which is very costly.

Further, gear and bearing devices are being developed and applied to newtechnologies that present additional challenges. For example, windenergy is a promising and fastest growing power generation source thatrelies heavily on gear and bearing devices with specific reliabilityneeds. An increase in the number of utility scale wind plants haveincreased the focus on the high operation and maintenance costs of windturbines as these ultimately impact the cost of wind energy. The drivetrain and actuators of wind turbines are major sources of failuresarising from the variability of wind, torque reversals, fluctuation inenergy demands, misalignment, and harsh environment conditions. Bearingsand gears in wind turbine drive trains suffer from failure modes likemicropitting, scuffing, spalling, and smearing, although these elementswere designed to meet twenty year service lives assuming that properlubrication and maintenance practices, and especially no unusual loadswere encountered. If a bearing has a low concentration of non-metallicinclusions in the steel, operates at the designed contact stress, andmaintains an adequate lubricant film thickness in the contact, then endof service life will be due to sub-surface originated spalling. Surfaceoriginated fatigue or pitting is caused by surface or near surfacestress risers such as non-metallic inclusions, plastically deformedmaterial, martensite transformation products, or several other factors.A particular type of surface initiated fatigue is known as micropittingwhich is a common failure mode encountered by gears and bearings.Specifically, many main shaft spherical roller bearings in wind turbinesare life limited due to spalls arising from micropitting wear.Micropitting is associated with the initiation and propagation ofmicro-cracks in the direction of traction forces. The progression ofmicro-pits alters the surface profile of a bearing raceway or gear toothwhich generates regions of large stress concentrations. The increase inlocalized stresses leads to fatigue failure through the formation ofmacro-pits or spalls. Micropitting is affected by several factors suchas lubricant type, contaminants, temperature, contact stresses,hardness, sliding speed, and surface roughness.

Studies were carried out over the last few decades to understand themechanism of micropitting. According to Morales-Espejel and Brizmer,micropitting depends on the lubrication conditions and roughness of thecontacting surfaces, the presence of slip (between 0.5 and 2%), and theassociated boundary friction shear stress are required for thegeneration of micropitting. Oila and Bull suggested that contact stresshas the greatest impact on micropitting initiation, while theprogression of micropitting is affected mostly by speed and slide toroll ratio. Lubrication conditions are best quantified by the parameterlambda (A), which is the ratio of the lubricant film thickness to thesquare root of composite surface roughness. Operating temperature,viscosity, and operating speed all affect the lubricant film thicknessand hence A. Brechot et al reported that oils with antiwear and extremepressure additives that are used to prevent scuffing and wear canpromote micropitting. Micropitting has proven to be difficult toeliminate through lubricant chemistry alone.

A number of solutions have been suggested to mitigate micropitting.Super-finishing is a process used on gear teeth to increase load bearingarea and reduce the severity of asperity interactions in boundarylubrication (i.e., λ<1). Apart from super-finishing, other surfaceengineering techniques are also employed to reduce asperity contact andprovide barriers to wear. Physical vapor deposition (PVD) coatingscomposed of nitrides, sulfides and carbides were examined for theirability to prevent micropitting. PVD coatings can be very effective atreducing or eliminating many wear modes. Among these coatings, diamondlike carbon (DLC) coatings are now being used in numerous applicationsfor wear resistant purposes due to their desirable tribologicalperformance. DLC has been modified over the years to possess ultra-lowfriction and high wear resistance. DLC coatings can be doped or alloyedto increase their functionality. The properties (hardness, toughness,thermal stability) of DLC coatings are further increased by using novelcoating architectures that consist of nanocrystalline precipitates andnanosized multilayers. Hydrogen-free DLC coatings deposited from solidcarbon targets can be extremely hard, while hydrogenated DLCs areusually much softer. In this research, coatings having indentationhardness values greater than 10 GPa are referred to as hard coatings,while coatings with indentation hardness values less than 10 GPa arereferred to as soft coatings. Precursor hydrocarbon gases such asmethane and acetylene are typically used in the deposition of DLC thatcontain large amounts of hydrogen. Hard DLC have been shown to be verysuccessful at mitigating many wear issues encountered by bearings andgears operation in boundary lubrication, including micropitting. Surfacetreatments such as black oxide and phosphate conversions are alsoapplied to bearings and gears to address micropitting. These conversionsare thick, sacrificial layers that work to rapidly break-in the surfacesof the components, reducing asperity contact, and delaying the onset ofmicropitting. Most of the studies reported on exploring the use of DLCto mitigate micropitting prevention were carried out with hard DLCcoatings.

SUMMARY OF THE INVENTION

One embodiment of the invention relates to an article of manufacture.The article comprises a metallic substrate. A carbide bonding layer isdeposited the metallic substrate. A hydrogenated diamond-like coating isdeposited on the carbide bonding layer. The hydrogenated diamond-likecoating has a hardness of 2-7 GPa and an elasticity of equal to or lessthan 60 GPa.

Another embodiments relates to a method of making a pitting resistantcarbon coating. The method comprises removing organic material from ametallic substrate; etching the surface of the metallic substrate;depositing a carbide forming layer on the etched metallic substrate;exposing the carbide forming layer to methane gas, forming a carbidelayer; and depositing an amorphous hydrogenated diamond-like coating onthe carbide layer. The hydrogenated diamond-like coating has a hardnessthe range of 2-7 GPa and an elasticity equal to or less than 60 GPa.

Another embodiment relates to a composition comprising a hydrogenateddiamond-like coating deposited on a substrate. The hydrogenateddiamond-like coating has a hardness the range of 2-7 GPa and anelasticity equal to or less than 60 GPa.

Additional features, advantages, and embodiments of the presentdisclosure may be set forth from consideration of the following detaileddescription, drawings, and claims. Moreover, it is to be understood thatboth the foregoing summary of the present disclosure and the followingdetailed description are exemplary and intended to provide furtherexplanation without further limiting the scope of the present disclosureclaimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects, features, and advantages ofthe disclosure will become more apparent and better understood byreferring to the following description taken in conjunction with theaccompanying drawings, in which:

FIG. 1A shows the micropitting rig test chamber. FIG. 1B shows thearrangement of rings and roller inside the test chamber of FIG. 1A. Theroller had a 10° chamfer on each side of a 1 mm track width.

FIGS. 2A and 2B show an optical image of the cylindrical roller and thetrack width prior to testing, where FIG. 2A is AISI 52100 steel rollerspecimen with dimensions and FIG. 2B is surface topography and trackwidth of the roller prior to testing.

FIG. 3 shows Raman spectra of Steel roller (top) H-DLC coated roller(middle) and PAO base oil (bottom).

FIG. 4 shows the nano-indentation measurements on the H-DLC coatingswith elastic modulus and hardness as a function of indentation depth ofthe coating.

FIGS. 5A-5D show Traction coefficient and P/P acceleration (vibration)as a function of contact cycles on the roller. Images show the surfaceof the roller after testing. FIG. 5A illustrates traction coefficientand P/P acceleration for steel roller and steel ring. FIG. 5Billustrates traction coefficient and P/P acceleration for H-DLC rollerand steel rings. FIG. 5C illustrates traction coefficient and P/Pacceleration for steel roller and H-DLC rings. FIG. 5D illustratestraction coefficient and P/P acceleration for H-DLC rings and H-DLCroller pairs.

FIGS. 6A-6D show high magnification topographical images of rollersurfaces after testing. FIG. 6A shows uncoated roller on uncoated rings.FIG. 6B shows H-DLC-coated roller on uncoated rings. FIG. 6C shows anuncoated roller on H-DLC-coated rings. FIG. 6D shows H-DLC-coated rolleron H-DLC-coated rings.

FIG. 7 shows the change in roller track width before and after testing,indicating the amount of wear that occurs on the roller sample

FIG. 8 shows Raman spectra of rollers after test completion for uncoatedroller on uncoated ring (top), H-DLC coated roller on uncoated rings(top-middle) uncoated roller on H-DLC coated rings (bottom-middle), andH-DLC coated roller on H-DLC coated rings material pairs (bottom).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof. In the drawings,similar symbols typically identify similar components, unless contextdictates otherwise. The illustrative embodiments described in thedetailed description, drawings, and claims are not meant to be limiting.Other embodiments may be utilized, and other changes may be made,without departing from the spirit or scope of the subject matterpresented here. It will be readily understood that the aspects of thepresent disclosure, as generally described herein, and illustrated inthe figures, can be arranged, substituted, combined, and designed in awide variety of different configurations, all of which are explicitlycontemplated and made part of this disclosure.

While prior art coatings have focused on hard coating materials toimprove gear and bearing surfaces, embodiments of the present inventioncomprises a high elasticity and low hardness coating. In one embodiment,the coating is a soft (2-7 GPa), highly hydrogenated diamond likecoating (“H-DLC”). In one embodiment, “soft” means about 2 GPa to about7 GPa, preferably 5 GPa to 6 GPa). In one embodiment, “highlyhydrogenate” means about 30% to 60% bonded or unbonded hydrogen. As theterm “diamond like coating” is used herein, it refers to an amorphouscarbon coating deposit by plasma based, plasma vapor deposition orchemical vapor deposition (PVD or CVD). The coating is preferably usableon a metal surface, such as a gear or bearing. Examples described hereinmay use a general metal surface in place of a gear or bearing ofillustrative purposes as a surrogate. “Highly Elastic” as used herein incertain embodiments means a coating having a Young's Modulus of 60 GPaor lower, preferably between about 50 GPa and 60 GPa. In one embodiment,the coating is highly elastic (60 GPa) and soft (6 GPa) with a highersp2 content (>60%) in comparison to previous NFC coatings.

In one embodiment, a 1/10 ratio, the penetration depth of the indentor(to measure hardness of coating) to the thickness of the coating, isused to avoid influence of the mechanical properties of the substrate.Unlike hard and/or inelastic coatings in the prior art, the presentcoatings do not exhibit a loss of adhesion (delamination).

In one embodiment, a method of forming the coating includes high powerimpulse magnetron sputtering (HiPIMS) method. In a preferred embodiment,the surface to be coated is steel. First the surface to be coated isetch to remove contamination layer and improve bonding, followed bymetallic bonding layer (such as chromium) applied using HiPIMS.Introduction of methane gas creates a metallic carbide layer on the topof the metallic bonding layer which serves as a second bonding layerwith good adhesive properties. Further the methane/Argon gas flow andsolid graphite source using DC magnetron sputtering produces anamorphous hydrogenated carbon coating which contains sp2 and sp3 bondedcarbon atoms on the metallic carbide layer. The combination of themethane/Argon gas and a graphite solid target with a DC power appliedproduces the characteristic soft quality of these coatings. The coatingthus obtained has an amorphous carbon layer that provides low friction(in one embodiment, less than about 0.1 coefficient of friction inlubricated conditions), high wear resistance (more than twice the wearresistance compared to uncoated in current test conditions and yetextreme resistance to cyclic loads and therefore, preventsmicro/macro-pitting.

Experimental results, described further below, showed that the uncoated,steel on steel, material pair failed after 32 million cycles due toexcessive surface damage. Large pits were observed on the roller due todamage accumulation and caused the P/P acceleration to exceed thecut-off limit. Based upon the observations of Fajdiga et al., thesurface damage on the roller appears to have initiated with micropits,evolved into macropits, that coalesced and formed the continuous surfacedamage in the wear track. Remarkable reductions in surface damage of theroller were observed when one or both elements were coated with anembodiment of the H-DLC. Traction coefficients were ˜0.04 and remainedrelatively constant throughout the testing for the three cases where atleast one of the contacting surfaces was coated. Furthermore, nofailures were observed and the tests were suspended after ˜100 millioncycles. It is believed that the H-DLC described herein is a promisingsurface treatment candidate to mitigate fatigue-initiated micropittingwear on surfaces of components operating in low A ratio rolling andmixed mode contacts.

Friction reduction and wear protection are the primary motivations forusing coatings on bearings and gears. Surface modifications that usecoatings and thin films offer numerous technical advantages overuntreated materials. Under boundary conditions in wind turbine drivetrains, physical vapor deposition (“PVD”) coatings such as WC/a-C:H, andconversion coatings such as black oxide are widely used on the bearingsand gears to improve tribological performance. Although conversioncoatings are typically considered to be sacrificial and used primarilyto prevent adhesive wear damage during run-in, new evidence has beengenerated that suggests that black oxide surface treatments on theraceways and rollers of wind turbine gearbox bearings may delay theonset of another failure mode termed white etch cracking. Mahmoudi etal. and Evans et al. reported that black oxide is not an attractivecandidate for preventing scuffing wear arising from roller/racewayskidding in highly stressed, low A environments. Although a hardWC/a-C:H (14 GPa) coating applied to the rolling elements of bearingsfunctioned very well in mitigating micropitting, scuffing, and fatiguelife reduction from debris damage, fracture-type wear of the coating wasobserved to occur at high contact stress cycles and ±10% slide/rollratios. The WC/aC:H coating is believed to provide wear protection bypolishing the uncoated mating surface and form a barrier to the adhesiveinteractions of asperities. Both of these proposed mechanisms have apositive effect on fatigue wear by increasing the A ratio andtransitioning the elastohydrodynamic lubrication (“EHL”) regime fromboundary to mixed.

Fewer investigations have been performed on the abilities of softcoatings to improve fatigue performance. Moorthy et al compared therolling contact fatigue performance of a soft Nb—S coating to a hardWC/a-C:H coating. Although the Nb—S coating exhibited no polishingeffect during the tests, no significant micropitting was observed.Clearly soft coatings like Nb—S, Ti—MoS₂, and H-DLC must functiondifferently from hard coatings like WC/a-C:H to provide fatigue lifeimprovements to steel components. Furthermore, since black oxide surfaceconversions do not mitigate micropitting (surface fatigue wear), thesoft coatings must also function differently from a soft black oxide.

Where a coated surface acts against (or is acted against) an uncoatedsurface, a transfer layer of H-DLC on the uncoated surface may form.Carbon from the H-DLC forms an amorphous carbon tribofilm containingiron oxide on uncoated steel surfaces during testing. This is supportedby the Raman spectrum of the tribofilm formed on the uncoated roller(H-DLC ring/uncoated roller pairing) shown in FIG. 8. The spectrum showsthe D and G mode vibrational signature of amorphous carbon. The lowfriction of the amorphous carbon tribofilm may be sufficient to reducethe magnitude of shear stresses generated on the roller surface duringtesting. Morales-Espejel and Brizmer point out that the presence of slipand the associated boundary friction shear stress are required for thegeneration of micropitting. Lubricant additives or low friction coatingsthat can reduce boundary friction shear stress should delay or mitigatemicropitting. A friction coefficient between 0.001-0.005 and wear ratesbetween 10¹¹-10¹⁰ mm³/Nm have been measured on H-DLC films in vacuumenvironments. Although black oxide surface treatments can obtain smoothinterfaces, the magnitude of the measured traction forces with smallamounts of slip are not as small as those obtained with the H-DLCcoating. Therefore it is concluded that the shear stresses between blackoxide treated surfaces are still large enough to initiate micropittingin boundary lubrication contact.

EXAMPLES Coating on AISI 52100 Steel

A H-DLC coating was deposited on AISI 52100 steel specimens using amagnetron sputter deposition system were tested using aMicro-Pitting-Rig (MPR) at 1.8 GPa contact stress, 40% slide to rollratio in polyalphaolefin base (PAO) oil. The post-test analysis wasperformed using optical microscopy, surface profilometry, and Ramanspectroscopy. The results showed a great potential for these coatings insliding/rolling contact applications as no failures were observed withcoated specimens even after 100 million cycles compared to uncoated testpairs which failed after 32 million cycles. The elastic modulus of thecoating is approx. 50-60 GPa, which is ¼th of the elastic modulus(Young's Modulus) of steel 210 GPa, and the coating has a hardness of 6GPa compared to 9 GPa of steel.

Test Apparatus

A PCS Instruments Micropitting Rig (MPR) was used for testing. The MPRis a computer controlled three rings on roller tribometer. A 12 mmdiameter roller is mounted in the center and in contact with three ringsof 54 mm diameter at an angle of 120°. FIG. 1A shows the MPR testchamber and the arrangement of rings and roller inside the test chamberis shown in FIG. 1B. A thermocouple was installed to measure the contacttemperature and an external cooler was connected to control thetemperature of the oil inside the test chamber. A load was applied tothe top ring (0° position) by means of motorized ball screw, andvibration was measured with a piezoelectric accelerometer. The rig has acapability to control entrainment speed (0-4 m/s), slide to roll ratio(0 to 200%), temperature (25-135 C), and load (0 to 1250 N).

Test Material

The test material used in this study and their properties are given inTable 1. The roller had a 10° chamfer on each side of a 1 mm track widthas shown in FIG. 2A. Rollers and rings were made of AISI 52100 steel andheat-treated to hardness values of 57-60 HRC and 62-65 HRC,respectively. The average surface roughness measured on the ring wasabout R_(a)=0.3 μm and roughness of the rollers was about R_(a)=0.2 μm.FIG. 2A shows an optical image of the cylindrical roller and the trackwidth prior to testing.

TABLE 1 Test Material and Properties Specimen Hardness Roughness,Diameter, Type Material (HRC) μm mm Rings AISI 52100 Steel 63-65 0.254.15 Roller AISI 52100 Steel 57-60 0.3 12

Table 2 presents the test parameters used for evaluating thetribological performance of different material combinations. Anunadditized polyalphaolefin base stock oil of viscosity grade 4 was usedas the lubricant, which was used to eliminate the contribution ofadditives on the performance of the coatings and also to ensure that asevere boundary regime was in place. Tests were performed at a 430 Nload, 3 ms⁻¹ speed, 40% slide-to-roll ratio (SRR) and at a constantoperating temperature of 55° C. The λ values were estimated to be 0.36which confirms direct metal-to-metal contact.

TABLE 2 Test Parameters Slide to Hertzian Roll Temper- Force, Stress,Speed, Ratio ature, N GPa Lambda, λ m/s (SRR) ° C. PAO4 430 N 1.8 0.36 340% 55

Rollers and rings were coated with the highly hydrogenated diamond-likecarbon (H-DLC) coating. Prior to coating, specimens were ultrasonicallycleaned using a solvent and dried in hot air before mounting on fixturesinside the deposition chamber. A pulsed magnetron sputtering system wasused for the deposition of the coatings using two carbon targets. Thepower supplied to carbon targets was in the range of 1000-2000 W andpulsed DC bias of −35 to −50 volts was supplied at 250-350 KHz. Methane(CH₄) and Argon (Ar) gas mixtures were maintained at 8-20 sccm and 70sccm, respectively. The final coating architecture was comprised of asteel substrate, followed by a thin Cr adhesion layer and top ˜1 μmhydrogenated DLC layer.

Rolling element bearings operating in wind turbine gearboxes employcoatings that are either applied only to rolling elements (hard DLC) orboth rolling elements and raceways (i.e., black oxide). The materialpairs tested in this study were designed to be consistent with theapplication of coatings on wind turbine bearing components. Theperformance of the H-DLC coatings was examined by testing four materialpairs: uncoated roller on uncoated rings, H-DLC coated roller onuncoated rings, uncoated roller on H-DLC coated rings, and H-DLC coatedroller on H-DLC coated rings. The uncoated on uncoated pairing was usedas the baseline in comparison with the other materials pairs. A value of1200 Peak/Peak acceleration (vibration) was used as the cut-off limit inthe tests to determine the cycles to failure. The 1200 P/P accelerationwas a vibration reading from the accelerometer that was placed close tothe contact zone and provided a view toward the progression of surfacedamage. Tests were suspended if the vibration exceeded the cut-off limitor exceeded 100 million contact cycles.

Characterization and Performance

Specimens were characterized by white light interferometry, opticalmicroscopy, and Raman spectroscopy, prior and subsequent to testing. ABruker 3D optical profilometer was used to measure the surface roughnessand surface topography of specimens. A Renishaw green light Ramanspectrometer with a wavelength of 633 nm was used to probe thestructural chemistry of the coated and uncoated specimens both prior toand after testing. Raman Instrument was calibrated using an internalsilicon reference, and the spectra were recorded in the range of100-4000 cm⁻¹. Raman spectra shown in FIG. 3 were obtained from anuncoated steel roller, and untested H-DLC-coated roller, and the PAO-4base oil. Both steel and the as-deposited H-DLC show a broad andfeatureless spectrum that is typical for the materials. The PAO-4 baseoil shows a strong characteristic feature at 2800 cm⁻¹ and few smallpeaks between 1000 cm⁻¹ and 1500 cm⁻¹.

Mechanical properties of the coating were measured by nanoindentationusing a Hysitron Triboindenter TI—950 equipped with a Berkovich diamondprobe and loads in the range of 0.5 mN to 12 mN. FIG. 4 shows thenano-indentation measurements on the H-DLC coatings. Nanoindentationmeasurements revealed that the hardness and elastic modulus values ofthe coating are about 6±1 GPa and 55±10 GPa, respectively.

Traction coefficients and P/P acceleration are plotted against contactcycles and are shown in FIG. 5 for the four materials pairs. Testparameters were Load=430N, SRR=40%, Oil=PA04, Temp.=55° C., Lambda=0.36.Also shown in the FIG. 5 are images of the roller wear tracks from eachtest. Vertical lines in the plot indicate instances where tests werestopped, the roller surface was inspected, and then the test wasrestarted. FIG. 5A shows the measurements obtained from the uncoated onuncoated pairing. Whereas the traction coefficient remainedapproximately constant at ˜0.05 through the test, the P/P accelerationtrace indicates that surface damage initiated at the beginning of thetest and gradually increased until about 28 million cycles. Thereafter,the P/P acceleration rapidly increased and exceeded the cut off limit atabout 31 million cycles. The image of the roller surface shows a largeamount of damage. A close inspection of the image coupled with the P/Pacceleration data suggests that micropits formed rapidly on the rollersurface, grew in size, and coalesced to produce an almost continuouslydamaged surface within the wear track.

FIG. 5B shows the traction coefficient and P/P acceleration as afunction of number of contact cycles on the roller for the H-DLC-coatedroller on uncoated rings pairing. This test achieved 100 million cycleswithout exceeding the P/P acceleration limit. The traction coefficientremained steady at ˜0.05 for first 40 million cycles and then began todecrease gradually until about 65 million cycles, and then remainedsteady for the rest of period at ˜0.035. The P/P acceleration wasinversely correlated with the traction coefficient. The optical image ofthe roller wear track after test termination is shown on right, and theimage shows that small regions of the coating delaminated. Since thecoating delamination could account for an increase in the P/Pacceleration, it is speculated that the decrease in the tractioncoefficient may also be associated in some way with the coatingdelamination.

FIG. 5C shows the traction coefficient and P/P acceleration as afunction of number of contact cycles for the pairing of the uncoatedroller and H-DLC-coated rings. No changes in the traction coefficient orP/P acceleration were observed in the test. The traction coefficient wasmeasured to be about ˜0.04 and the P/P acceleration value was ˜200 aftercompleting 100 million cycles. The image of the uncoated steel rollershown on the right indicates that no micro or macro pits formed duringtesting and that a tribofilm was generated over a large region of thewear track.

FIG. 5D shows the traction coefficient and P/P acceleration as afunction of number of contact cycles for the pairing of the H-DLC-coatedroller and H-DLC-coated rings. The test run for 100 million cycles. Thetraction coefficient was constant at ˜0.04 throughout the test while theP/P acceleration increased slightly after about 45 million cycles from avalue of 200 to 300 and remained at this value until the end of test.Thus, a steady state appear to be reached. The image of the wear trackon the roller contains a few pits, but no significant damage wasobserved.

All the tests where at least one surface was coated with the H-DLClasted for 100 million cycles without experiencing a significant amountof surface damage. FIGS. 6a-d show high magnification topographicalimages of roller surfaces after testing for (a) uncoated roller onuncoated rings (b) H-DLC-coated roller on uncoated rings (c) uncoatedroller on H-DLC-coated rings, and (d) H-DLC-coated roller onH-DLC-coated rings.

Roller wear was quantified according to a change in width of the rollerwear track. An optical microscope was used to measure the track width atmultiple locations. Values shown in FIG. 7 are averages and standarddeviations of four measurements of the change in roller track width. Thedata shown in the figure was normalized based on the number of contactcycles. The calculated track width was divided by the total no contactcycles after failure or after termination. The total change in width ofwear track was below 50 μm in all tests but the uncoated pairing whichshowed the largest change in track width. A clear trend is seen betweenchange in track width and the amount of coating available to participatein the tribological contact. For example, the least amount of coatingpassing through the contact was for the H-DLC-coated roller on uncoatedring pair, while the largest amount of coating passing through thecontact was for the H-DLC-coated roller on H-DLC-coated ring pair,considering the difference in coated sample surface area. This trendsuggests that the wear occurring on the roller depended upon the amountof H-DLC present in the contacts. In the current study 1 um thicknesswas used, but the thickness can reasonably range from 0.1-10 um}

Raman spectra obtained on the tested specimens are shown in FIG. 8. Nosignificant differences are observed from spectrum of the untestedroller in FIG. 3 and the spectrum of the steel roller after testing. TheRaman spectrum from the H-DLC-coated roller on uncoated rings pairingshows D and G peaks at around 1332 cm⁻¹ and 1580 cm⁻¹, respectively,which are typical of DLCs with high sp² bond characters. A minor peakaround 600 cm⁻¹ corresponds to α-Fe₂O₃ that was probably generated fromasperities on the rings. The Raman spectrum of the tribofilm formed onthe roller during the uncoated roller on H-DLC-coated ring test showssimilar D & G features, a strong peak at 670 cm⁻¹ from Fe₃O₄ or FeO, andminor peaks around 222 cm⁻¹ and 298 cm⁻¹ from α-Fe₂O₃. Since thetribofilm formed on the roller in the uncoated roller on H-DLC coatedring test has similar D & G features in its Raman spectrum as the Ramanspectra of the wear tracks from the H-DLC-coated roller on H-DLC-coatedring and H-DLC-coated roller on uncoated ring testing, there is a strongindication that the H-DLC coating undergoes a transition from itsinitial amorphous hydrocarbon state to disordered graphite within thewear track and then transfers from the coated rings to the wear track onthe uncoated roller.

CONCLUSIONS

This research evaluated the tribological performance of highlyhydrogenated diamond like carbon films in mixed rolling and slidingcontacts for bearings and gears in wind turbine drive train. H-DLC filmswere deposited on cylindrical specimens and compared against untreatedsamples using PAO 4 synthetic base oil as a lubricant. H-DLC coatedsamples provided significant improvement in mitigating surface fatigue(micropitting) compared to uncoated steel samples. The results concludedfrom the experimental investigation are as follows:

-   -   Nano-indentation results revealed that the coatings are highly        elastic in nature and have hardness values (˜6 GPa) comparable        to that of steel substrate.    -   Uncoated steel/steel pairs failed after 32 million cycles        whereas no failure was observed with soft-highly hydrogenated        diamond like carbon coated test samples up to 100 million        cycles.    -   Roller track width measurements revealed no significant change        in the track at the end of test compared to initial track width.    -   Raman analysis showed microstructural transformations of the        H-DLC inside the roller wear track.    -   Results indicate that coating only one side of the material pair        is sufficient to delay the onset of surface fatigue of test        rollers by more than 100 million cycles. H-DLC appears to be an        effective solution for components that suffer from surface        initiated fatigue.

In one embodiment, the H-DLC improves wear and performance on rollingand sliding surfaces resulting in improved reliability, energy savings,and maintenance of developed systems. In one embodiment, the coating isused on gear boxes, such as for wind turbines.

The foregoing description of illustrative embodiments has been presentedfor purposes of illustration and of description. It is not intended tobe exhaustive or limiting with respect to the precise form disclosed,and modifications and variations are possible in light of the aboveteachings or may be acquired from practice of the disclosed embodiments.It is intended that the scope of the invention be defined by the claimsappended hereto and their equivalents.

What is claimed is:
 1. An article of manufacture comprising: a metallicsubstrate; a carbide bonding layer deposited the metallic substrate; anda hydrogenated diamond-like coating deposited on the carbide bondinglayer; wherein the hydrogenated diamond-like coating has a hardness of2-7 GPa and an elasticity of equal to or less than 60 GPa.
 2. Thearticle of manufacture of claim 1, further comprising an amorphouscarbon tribofilm.
 3. The article of manufacture of claim 1, wherein thetribofilm comprises iron oxide.
 4. The article of manufacture of claim1, wherein H-DLC has between 30% to 60% bonded or unbonded hydrogen. 5.The article of manufacture of claim 1, wherein the metallic substrate isa rolling element.
 6. The article of manufacture of claim 1, wherein themetallic substrate is a track member.
 7. The article of manufacture ofclaim 1, wherein the hardness is 5-6 and an elasticity of between 50 GPaand 60 GPa.
 8. A method of making a pitting resistant carbon coatingcomprising: removing organic material from a metallic substrate; etchingthe surface of the metallic substrate; depositing a carbide forminglayer on the etched metallic substrate; exposing the carbide forminglayer to methane gas, forming a carbide layer; and depositing anamorphous hydrogenated diamond-like coating on the carbide layer;wherein the hydrogenated diamond-like coating has a hardness the rangeof 2-7 GPa and an elasticity equal to or less than 60 GPa.
 9. The methodof claim 8, wherein the metallic substrate is steel.
 10. The method ofclaim 9, further comprising forming an amorphous carbon tribofilm. 11.The method of claim 9, wherein the deposited amorphous hydrogenateddiamond-like coating has a thickness at least 1/10^(th) a thickness ofthe metallic substrate.
 12. The method of claim 8, wherein thehydrogenated diamond-like coating has a hardness the range of 5-6 GPaand an elasticity between 50 and 60 GPa.
 13. The method of claim 8,wherein the H-DLC is deposited by a pulsed magnetron sputtering system.14. The method of claim 13, wherein the pulsed magnetron deposition usespower supplied to carbon targets was in the range of 1000-2000 W andpulsed DC bias of −35 to −50 volts was supplied at 250-350 KHz.
 15. Themethod of claim 14, wherein the methane was maintained at 8-20 sccm. 16.The method of claim 15, wherein the deposition by pulsed magnetron usesargon gas mixture maintained 70 sccm as a working gas.
 17. A compositioncomprising: a hydrogenated diamond-like coating deposited on asubstrate; and wherein the hydrogenated diamond-like coating has ahardness the range of 2-7 GPa and an elasticity equal to or less than 60GPa.
 18. The composition of claim 17, wherein the metallic substrate issteel.
 19. The composition of claim 17, further comprising an amorphouscarbon tribofilm.
 20. The composition of claim 17, wherein the tribofilmcomprises iron oxide.